The present invention relates to a sensor, in particular to an electrochemical sensor.
Electrochemical sensors of the generic type are known. These conventional electrochemical sensors include an electrochemical element which has an electrochemical pump cell with a preferably flat first solid-electrolyte body and a first and second preferably porous electrode. These sensors further include an electrochemical sensor cell, connected to the pump cell, with a preferably flat second solid-electrolyte body and a third and fourth preferably porous electrode, as well as a diffusion resistor arrangement connected to the sampled gas chamber via a gas inlet opening and a gas inlet duct and surrounded by the two solid-electrolyte parts, i.e. located in an inner cavity. The diffusion resistor arrangement can be filled with a porous substance. The sampled gas enters the inner cavity via the gas inlet opening and the gas inlet duct, with the first and second electrodes of the pump cell regulating the entry of the sampled gas into the cavity, thereby producing a controlled partial pressure on the gas component to be measured. Power is supplied to the electrochemical pump cell by a device mounted outside the electrochemical element.
Due to the different partial pressures of the gas in the diffusion resistor arrangement and in a reference gas chamber located, for example, in the second solid-electrolyte body, an electrochemical potential difference occurs between the electrodes of the second solid-electrolyte body and is measured by a voltmeter unit positioned outside the electrochemical element.
Another coventional method provide the electrochemical sensor with an electrical heater which heats both the electrochemical pump cell and the electrochemical sensor cell in order to ensure a suitable operating temperature for the electrochemical sensor.
The layout of the electrochemical sensor known from conventional methods has a disadvantage in that liquid components contained in the sampled gas, for example drops of gasoline in the exhaust gas of an internal combustion engine, and solid components, for example particles of soot, can enter the inner cavity through the gas inlet opening of the electrochemical sensor and interfere with the function of the electrochemical sensor over a long period of time. The measured value determined by the electrochemical sensor can be corrupted by exhaust gas that has been enriched with gasoline (xe2x80x9crichxe2x80x9d exhaust gas). Clogging of the gas inlet opening can even cause the electrochemical sensor to break down.
The present invention relates to an electrochemical sensor for measuring a gas concentration, for example an oxygen concentration, of a sampled gas which has an electrochemical element, including an electrochemical pump cell with a first and a second electrode and with an inner gas chamber which is connected to the sampled gas via a gas inlet opening and located in one of the two electrodes, with the gas inlet opening being covered by a porous covering. The present invention offers the advantage of preventing liquid and solid components contained in the sampled gas from penetrating the interior of the sensor, i.e. the inner cavity referred to as the gas chamber. This is done by applying a porous layer to the surface of the electrochemical element facing the sampled gas chamber as a covering for the gas inlet opening. This covering is permeable to the sampled gas, yet presents a barrier to the liquid and solid components contained in the sampled gas. The liquid held back by and deposited in this covering, for example gasoline, quickly evaporates after a heater, which is preferably provided, is turned on, so that only gasoline vapor is forced out of the interior of the electrochemical sensor by the gas, for example oxygen, which is continuously being pumped by the electrochemical pump cell.
The present invention also relates to a method for producing this type of electrochemical sensor in which the gas inlet opening is first created and subsequently covered with a covering layer. The porous covering is finally applied to this additional covering layer. This advantageously ensures that the application of a porous covering does not impair the porosity characteristics and thus the operability of the electrochemical sensor.
The covering layer can itself be porous, i.e. permeable to the sampled gas. However, materials that burn without leaving residue under heat treatment or pore-forming materials are advantageously used for the covering layer. If pore-forming materials are used, an additional protective membrane remains after producing the electrochemical sensor according to the present invention, preferably after sintering. Suitable materials include wax, carbon black, graphite, methyl xanthines such as theobromine, theophylline or caffeine. The covering layer is preferably applied by a transfer technology, while the porous covering is preferably applied by screen printing.
According to a further advantageous embodiment of the present invention, a hollow space is created in the gas inlet duct which is advantageously partially filled with a porous substance, i.e. between the covering provided on the gas inlet opening and a porous filling advantageously located in the inner cavity. The hollow space prevents capillary migration of the liquid gasoline from the covering according to the present invention to the inner porous filling. This hollow space, which thus forms a barrier between the porous covering and the porous filling located in the inner cavity, can be preferably formed by burning out sublimatable material during sintering. The covering on the gas inlet opening is preferably made of a porous material which can be a continuation of the porous protective layer covering the entire surfaces of the electrochemical element facing the sampled gas chamber.
In to another advantageous embodiment, the present invention relates to dual-cell sensors (broad-band sensors) which have a pump cell, including a first solid-electrolyte body, and concentration cell, including a second solid-electrolyte body.